Graphic films are used for example to apply designs, e.g. images, graphics, text and/or information, on windows, buildings, pavements or vehicles such as autos, vans, buses, trucks, streetcars and the like for e.g. advertising or decorative purposes. Many of the surfaces of the mentioned substrates, e.g. vehicles, are irregular and/or uneven, such as rivets overlaying flat surfaces, corrugated panels, curvatures or various surface molding, e.g. body-side, structural or decorative molding.
For graphic films, it is desirable to have a film that accommodates curves, depressions or projections on a substrate surface so that the film may be stretched around curves or projections, or may be pressed down into depressions without breaking or delaminating the film (conformability). It is also desirable to have a film that can be applied over irregular and/or uneven surfaces without subsequent delaminating or releasing from the substrate surface (popping-up). In addition, it is desirable that graphic films are imageable (i.e. have a receptive surface for printing and/or graphics) and exhibit good weathering for outdoor applications.
Polyvinyl chloride (PVC) films are conventionally used for a wide of variety of applications including graphic films. PVC has many properties that are advantageous for such applications. For example, PVC graphic films are conformable to the varying topographies present on the exterior of a substrate, e.g. a vehicle. But PVC films and their use as graphic films suffer from a number of disadvantages. In general, the application of halogen-containing plastics is undesirable for environmental reasons. Another disadvantage of PVC films is the accompanying use of plasticizers in such films. Plasticizers are typically needed in PVC graphic films in order to make the films more flexible and more conformable. However, plasticizers can migrate within the film and such migration can change the performance of the film. Furthermore, such plasticizers may degrade adjacent adhesive layers, reducing the adherence of the film to the surface of the substrate. Another disadvantage of PVC graphic films is that they tend to pop up after application.
Although polyolefin films and their corresponding use as graphic films are advantageous in that they typically do not contain halogen or plasticizers, polyolefin films are difficult to image and thus require the application of an additional image receptive layer, such as an ink receptive layer, toner receptive layer, primer layer or the like. Furthermore, polyolefin graphic films like PVC graphic films show a tendency to pop up.
Popping-up typically results from the presence of residual stress in the graphic film at irregular surface locations during application, which exceeds the, holding power of the adhesive. In previous attempts to minimize the problem of popping up, graphic films have been provided with highly tacking adhesives to provide high holding power and/or applied to the substrate surface at relatively high temperatures well above 100° C. in order to try to fully relax the film.